Propeller Centerline of Thrust

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batesjoe

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Mar 21, 2013
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Alamogordo
In many homebuilt aircraft, engine choices can be quite varied, including those with reduction gears and those that are direct drive.

Aside from changes in mounts and cowling, from an aerodynamic point of view, how critical is propeller installation to the aircraft longitudinal waterline?

I already know about prop diameter & the tip strike potential, but am mostly wondering about the center of thrust as it relates to the aircraft waterline.

Thanks
 

Dana

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Within reason, small changes in the thrust line will not be significant. If you're trying to eke out every last bit of performance from a high efficiency ship, you might see some small effect on trim drag. When you put the engine way high up, like the common over the wing pusher installation, they you see significant pitch effects from power changes.

-Dana

Hardware: the part of the computer that can be kicked. If you can only curse at it, it's software.
 

Rick McWilliams

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Feb 27, 2011
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Vertical thrust line is not very critical with respect to performance. It is important with respect to trim. Amphibians with very high thrust lines usually have significant thrust angles and have the propwash include the horizontal stabilizer. My Mermaid amphibian has a high thrust line but almost zero trim change with power.
 

cheapracer

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Just to add one in here, I was at a well respected Oz aircraft manufacturer last month and a Rotax 914 turbo installation in a high wing LSA STOL was significantly to the left of center. The plane was to be used for mustering.
 

Dan Thomas

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The up/down and sideways angles of thrust in a normal configuration, not a high-mounted amphib prop, can make a difference in handling qualities. Cessna, for instance, has the 172's thrust line parallel to the waterline and has no lateral offset either. Piper, in their Cherokee line of airplanes, tilted the nose of the engine down a little and to the right. Both of these reduce the left-turning tendency in the takeoff and climb, at a small cost to cruise performance. The Ercoupe had its thrust line tilted down quite a bit to make it easier to fly.

When we were young we used to buy those plastic Cox PT-19 control-line models with the .049 glow engines in them. They had an engine-tilting option built into them, and if you were a greenhorn at control-line flying, you tilted the thrust line down. Made the airplane more sluggish and easier to control. Tilted it up once you got better at it.

So it depends what you want. Performance and efficiency, or a self-flying feet-on-the-floor airplane.

Dan
 

BBerson

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Just to add one in here, I was at a well respected Oz aircraft manufacturer last month and a Rotax 914 turbo installation in a high wing LSA STOL was significantly to the left of center. The plane was to be used for mustering.
Was that so the pilot could see around engine better?( Or some other reason)
 
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